1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of manufacturing the liquid crystal display device.
2. Description of the Related Art
An active matrix display is known as a liquid crystal display device technology which provides high-quality images. The active matrix display uses a field-effect thin film transistor (TFT) as a switching device of a pixel. The active matrix display has the features of thin profile, light weight and low power consumption and offers clear contrast and high-speed display. Therefore, the active matrix display is widely used as a monitor of portable computers, space-saving desktop computers and so on.
The creation of high-quality images requires improved viewing angle characteristics. As an effort to improve the viewing angle characteristics, an in-plane switching (IPS) active matrix liquid crystal display device is disclosed in Japanese Unexamined Patent Application Publication No. 2002-139727. The IPS active matrix liquid crystal display device uses a horizontal electric field that applies an electric field in the horizontal direction. However, in the IPS active matrix liquid crystal display device of the related art, “after-image” or “burn-in” effects occur to degrade the performance of the display. Specifically, when another display pattern is displayed after continuously displaying the same display pattern for a long time, the previous pattern remains persistently displayed together with the new pattern. Particularly, the after-image and burn-in effects can get worse if a display is stored for a long period of time during manufacture. The IPS active matrix liquid crystal display device thus has the problem of the occurrence of the after-image and burn-in effects.
The structure of an active matrix liquid crystal display device of a related art is described hereinafter with reference to FIG. 5. FIG. 5 is a cross-sectional view of an active matrix liquid crystal display device. As shown in FIG. 5, an electrode substrate 1 includes a pixel electrode and a plurality of other electrodes of a scan signal line, a video signal line and so on. A counter substrate 3 is placed opposite to the electrode substrate 1. The electrode substrate 1 and the counter substrate 3 are attached to each other by a sealing material 4. Liquid crystals 9 are interposed between the electrode substrate 1 and the counter substrate 3. The counter substrate 3 is coated with an overcoat layer 8 on the surface facing the electrode substrate 1. In the liquid crystal display device of a related art, the overcoat layer 8 spreads from a display area to the periphery of the panel, which is on the outward side of the sealing portion. The overcoat layer 8 is typically made of acrylic resin or epoxy resin. Thus, the overcoat layer 8 absorbs moisture over time at the portion in contact with the outside air, so that the moisture reaches the display area.
The overcoat layer 8 of the counter substrate 3 contains inorganic ion impurity and organic ion impurity. The inorganic ion impurity enters the overcoat layer 8 in the process of manufacture. The organic ion impurity is a by-product of the curing reaction during manufacture. Such impurity ion 13 is movable through the medium of the moisture which is absorbed from the outside air as above. If an electric field is applied externally in such a condition, the electric field is also oriented vertically to the counter substrate 3 as shown in FIG. 6. As a result, polarization occurs in the overcoat layer 8 as shown in FIG. 7. Due to the overcoat polarization, liquid crystals are aligned vertically with respect to the substrate surface, which causes the after-image and burn-in effects.
In view of the foregoing, it is an object of the present invention to provide a liquid crystal display device capable of providing high-quality display and a method of manufacturing the liquid crystal display device.